In recent years, small mobile products equipped with an imaging device (camera module), e.g., mobile phones, have become widely popular, and taking a picture instantly with the small mobile product has become a common practice. As a photography lens for a small imaging device incorporated in the small mobile product, a photography lens configured with three lens groups including: a first lens as a first lens group; a second lens as a second lens group; and a third lens as a third lens group, arranged in that order from an object side to an image surface side, has been becoming popular. This is because its performance can be enhanced further in comparison with a photography lens configured with one or two lenses (e.g., see Patent Documents 1 and 2).
The photography lens described in Patent Document 1 includes: an aperture stop; a first lens having positive refractive power whose lens surface facing the object side is convex (first lens group); a second lens having positive refractive power whose lens surface facing the object side is concave (second lens group); and a third lens having negative refractive power whose lens surface facing the object side is convex (third lens group), arranged in that order from the object side to the image surface side. The first lens is configured as a glass lens, and the second and the third lenses are configured as plastic lenses. The reason for using a glass lens as the first lens is to correct, in particular, longitudinal chromatic aberration and lateral chromatic aberration favorably.
However, as described above, since a glass lens is used as the first lens in the photography lens described in Patent Document 1, the manufacturing cost is increased. Further, the configuration described in Patent Document 1 results in a dark photography lens having a large F number.
The photography lens described in Patent Document 2 intends to solve the problems of the photography lens described in Patent Document 1, and includes: an aperture stop; a first lens having a double-convex shape and positive refractive power (first lens group); a second lens having negative refractive power whose lens surface facing the object side is concave (second lens group); and a third lens having a meniscus shape whose lens surface facing the object side is convex (third lens group), arranged in that order from the object side to the image surface side. The first to the third lenses are all configured as plastic lenses.
As described above, the photography lens described in Patent Document 2 includes the first to the third lenses that are all configured as plastic lenses, therefore suitable for a mass production at low manufacturing cost. However, aberrations cannot be corrected sufficiently only with the above lens configuration, and thus the photography lens is not compatible with a high pixel imaging element of a megapixel or more. Further, since the configuration described in Patent Document 2 results in an increase in the total length of the optical system, downsizing and thinning of the photography lens become difficult to accomplish.
For example, Patent Document 3 proposes a bright photography lens having a small F number that is compatible with a high pixel imaging element of a megapixel or more.
The photography lens described in Patent Document 3 includes: an aperture stop; a cemented lens made of a first lens having positive refractive power and a second lens having negative refractive power (first lens group); a third lens as a meniscus lens in which at least one of the lens surfaces is aspherical and the lens surface facing the object side is concave (second lens group); and a fourth lens as a meniscus lens in which at least one of the lens surfaces is aspherical and the lens surface facing the object side is convex (third lens group), arranged in that order from the object side to the image surface side (configured with three lens groups including four lenses). The first and the second lenses are configured as glass lenses, and the third and the fourth lenses are configured as plastic lenses.
Patent Document 1: JP 2004-212467 A
Patent Document 2: JP 2004-4566 A
Patent Document 3: JP 2003-255222 A